The present invention relates generally to composites having a titanium base matrix reinforced by silicon carbide fiber or filament reinforcement. More particularly, it relates to improvements in the filament and matrix components of a silicon carbide reinforced titanium aluminide composite.
The preparation of titanium alloy base foils and sheets and of reinforced structures in which silicon carbide fibers are embedded in a titanium base alloy are described in the patents: U.S. Pat. Nos. 4,775,547; 4,782,884; 4,786,566; 4,805,294; 4,805,833; and 4,838,337; assigned to the same assignee as the subject application The preparation of these composites is the subject of intense study inasmuch as the composites have very high strength property in relation to their weight. Prior to the development of the processes described in the above-referenced patents, such structures were prepared by sandwiching the reinforcing filaments between foils of titanium base alloy and pressing the stacks of alternate layers of alloy and reinforcing filament until a composite structure was formed. However, that prior art practice resulted in fracture of the reinforcing filaments to a degree that many of the beneficial advantages of the reinforced product were lost or were not achieved to the degree sought because of the filament fracture problem.
The structures taught in the above-referenced patents greatly improved over the earlier practice of forming sandwiches by compression.
It has been found that while the structures prepared as described in the above-referenced patents have properties which are a great improvement over earlier structures, the attainment of the potentially very high ultimate tensile strength in these structures did not measure up to the values theoretically possible.
The testing of composites formed according to the methods taught in the above patents has demonstrated that although modulus values are generally in good agreement with the rule of mixture predictions, the ultimate tensile strength is usually much lower than predicted by the underlying properties of the individual ingredients to the composite.
Further, for certain titanium base alloys, testing has shown that the total strain to composite fracture is relatively low and, in addition, extensive off-plane cracking of the matrix has been observed. I have found that the matrix in certain composites consists primarily of an alpha-2 crystal form which is an ordered intermetallic phase. The secondary constituent of the matrix is small amounts of beta-phase. This crystal form exists in alloys containing 14 weight % aluminum and 21 weight % niobium in a titanium base. The alpha-2 crystal material of such alloys tends to have low ductility and envelopes of this phase around the SiC fibers have been found to crack during consolidation of the matrix and fibers into a reinforced composite.
From my work and from the observations and analysis that have been made, I conceived the notion that modification of the phase distribution of the alloy in the matrix could contribute toward inhibiting matrix cracking and could result in property improvement.